The present application is directed to the art of monitoring physiological conditions within a high magnetic field environment. The present invention finds particular application in conjunction with pulse oximetry measurements within the bore of a magnetic resonance imaging apparatus and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with the measurement of other physiological conditions within MRI equipment or other equipment with strong, changing, magnetic fields.
During a diagnostic examination, physicians often want to monitor various physiological conditions. Among these conditions are the patient's pulse rate and blood oxygen content. Pulse oximetry equipment is commonly available off-the-shelf for measuring a patient's pulse and blood oxygen content. Typically, a pulse oximetry system includes a visible red and an infrared light source, such as a pair of light emitting diodes. The light sources and a light sensor are mounted in a sensor unit that is attached to the patient such that the light passes through a portion of the patient before reaching the light sensor. The amount of blood oxygen is readily determinable from the difference in the absorption of these two wavelengths of light.
Typically, a cable extends from the sensor unit to a processing and display unit. The processing and display unit provides electrical power along the cable for the light emitting diodes and the sensor. This unit further receives signals from the sensor and performs an appropriate analysis to determine the patient's blood oxygen content and pulse rate.
It has been proposed to clip the pulse oximeter sensor unit to a patient before placement in the bore of a magnetic resonance imager and position the processing and display unit adjacent the exterior of the bore where its display can be viewed by the doctor or attendant. However, this has several drawbacks. First, the processing and display units typically have an internal clock, e.g. a microprocessor clock, which has harmonics at or near the resonance frequencies of a magnetic resonance imager. An 0.5 Tesla MRI machine has about a 21 MHz resonance frequency; a 1.0 Tesla MRI machine has about a 42 MHz resonance frequency; and a 1.5 Tesla MRI machine has about a 64 MHz resonance frequency. The processing and display unit and the lead running to the sensor unit tend to broadcast radio frequency signals of the microprocessor clock frequency and its harmonics into the bore of the magnetic resonance imaging apparatus. These radio frequency signals interfere with the proper operation of the magnetic resonance imager, degrading the resultant images.
Second, the electrical leads extending from the processor and display unit to the sensor unit act as the secondary or pick-up coil when subject to the changing magnetic fields of the magnetic resonance imager. That is, the changing magnetic fields in the bore tend to induce like currents in the electrical leads extending between the sensor unit and the processing and display unit. These radio frequency currents in the lead can interfere with the proper processing of the pulse oximetry signals and even damage the processor and display unit. Yet more dangerous, these same induced RF currents can cause excessive electrical inductive and resistance heating, particularly adjacent the sensor unit. This can cause RF burns on the patient.
With these problems in mind, others have produced a pulse oximetry unit in which the sensor unit is connected with the processor and display unit by fiber optic cables. Because there are no electrical cables extending through the magnetic resonance imager bore, there is no possibility of RF burns to the patient or RF induced noise in the signal conveyed to the processor and display unit. However, the fiber optic leads tend to be relatively delicate and easily broken. Once damaged, the cost of repair is very high. Further, the fiber optic systems normally require different clips for patients of different size, particularly separate adult and pediatric clips or sensor units. Individual sensor units are again very expensive.
Another solution is to encase the sensor unit in a shielded mit. The mit and the cable are surrounded with a grounded shield. This system prevents RF currents from being induced on the conductors connecting the sensor unit with the processing and display unit, eliminating the interference which such RF currents can cause with both the signals coming to the processing unit and the signals going to the sensing unit. However, the RF magnetic fields can induce RF currents in the shielding which RF currents can still cause RF burns to the patient.
The present invention provides a new and improved combination MRI imaging apparatus and pulse oximetry system which overcomes the above-referenced problems and others.